With the recent advances in small-incision cataract surgery, increased emphasis has been placed on developing soft, foldable materials suitable for use in artificial lenses. In general, these materials fall into one of three categories: hydrogels, silicones, and acrylics.
In general, hydrogel materials have a relatively low refractive index, making them less desirable than other materials because of the thicker lens optic necessary to achieve a given refractive power. Silicone materials generally have a higher refractive index than hydrogels, but tend to unfold explosively after being placed in the eye in a folded position. Explosive unfolding can potentially damage the corneal endothelium and/or rupture the natural lens capsule. Acrylic materials are desirable because they typically have a higher refractive index than silicone materials and unfold more slowly or controllably than silicone materials.
U.S. Pat. No. 5,290,892 discloses high refractive index, acrylic materials suitable for use as an IOL material. These acrylic materials contain, as principal components, two aryl acrylic monomers. They also contain a cross-linking component. The IOLs made of these acrylic materials can be rolled or folded for insertion through small incisions.
U.S. Pat. No. 5,331,073 also discloses soft acrylic IOL materials. These materials contain as principal components, two acrylic monomers which are defined by the properties of their respective homopolymers. The first monomer is defined as one in which its homopolymer has a refractive index of at least about 1.50. The second monomer is defined as one in which its homopolymer has a glass transition temperature less than about 22° C. These IOL materials also contain a cross-linking component. Additionally, these materials may optionally contain a fourth constituent, different from the first three constituents, which is derived from a hydrophilic monomer. These materials preferably have a total of less than about 15% by weight of a hydrophilic component.
U.S. Pat. No. 5,693,095 discloses foldable ophthalmic lens materials comprising a total of at least 90% by weight of only two principal lens-forming monomers. One lens-forming monomer is an aryl acrylic hydrophobic monomer. The other lens-forming monomer is a hydrophilic monomer. The lens materials also comprise a cross-linking monomer and optionally comprise a UV absorber, polymerization initiators, reactive UV absorbers and reactive blue-light absorbers.
U.S. Pat. No. 6,653,422 discloses foldable ophthalmic lens materials consisting essentially of a single device-forming monomer and at least one cross-linking monomer. The materials optionally contain a reactive UV absorber and optionally contain a reactive blue-light absorber. The single device-forming monomer is present in an amount of at least about 80% by weight. The device-forming monomer is an aryl acrylic hydrophobic monomer.
Acrylic materials with a higher refractive index have historically been preferred as IOL materials because less material is required to make a lens of a given dioptric power. Thus a lens made from a higher refractive index material may be implanted through a smaller incision than a similar power lens made from a lower refractive index material. Use of a smaller incision in turn results in fewer traumas and reduces the likelihood of surgically induced astigmatism. However, polymer materials with high refractive index also generally exhibit refractive index dispersion. This can result in chromatic aberrations that may impact visual performance when viewing light of differing wavelengths.
In general, the presence of aromatic groups leads to materials with higher refractive index dispersion. Hydrophobic acrylic materials without aromatic groups will have reduced refractive index dispersion but will also have lower refractive index, thus will require a larger incision size than a comparable lens made from a high refractive index polymer.